1. Field of the Invention
This invention relates generally to apparatus for charging particles and more specifically to apparatus for high frequency A. C. field charging of aerosol particles for employment in an ink mist printing operation.
2. Prior Art
The charging of aerosol particles in an ink mist printing operation is usually accomplished by a D. C. charging electrode. In such a charging system, generally referred to as electrostatic charging, the aerosol particles are charged by ions which are drawn from a corona source by a D. C. charging electrode. An electric field is produced in the passage between the corona source and the D. C. charging electrode which effectively draws ions from the corona. The aerosol particles when passing through this passage concentrate the electric field lines such that ions are drawn to their surfaces.
While the electrostatic charging method described above produces a charged particle suitable for use in a printing operation, the practice of this method has been hampered by precipitation of charged particles to the D. C. charging electrode. As the particles are charged they are also attracted to the D. C. charging electrode. In other words, the same electric field which charges the particles also causes them to precipitate. This precipitation eventually begins to block the passage in which the charging is done, and may cause electrical shorts between the electrodes if the particles are electrically conductive. To reduce the amount of charged aerosol particles which precipitate to the D. C. charging electrode, guard flows have been used. Even though guard flows eliminate some precipitation in the D. C. charging system, they have proven to be very ineffective in eliminating a substantial portion of the precipitate to the charging electrode over extended periods of operation or charging.
The employment of A. C. and transient voltages in electrostatic precipitation operations is known. However, previously known methods and apparatus for electrostatically precipitating particles have utilized A. C. and transient voltages as charging sources for the corona to generate a more intense corona and thus produce pulses of ions to impart an electrical charge to the aerosol particles. It is also known in the electrostatic precipitation art that an R. F. ripple can be superimposed on the constant potential supplied to ionizing wires in the corona source. This has the effect of increasing corona emission to the charging region. The precipitation of aerosol particles in the examples mentioned above is not retarded, but is increased by superimposing A. C. and transient voltages on potentials applied to the corona source.
It is also known to use low frequency A. C. voltage sources to produce pulses of ions. When using low frequency A. C. sources, the aerosol particles are charged directly in the corona. This results in pulses of charged particles rather than a continuous stream of uniformly charged particles.